The present disclosure relates to optical fiber amplifiers. Conventional optical fiber communications typically use dense wavelength division multiplexing (DWDM). DWDM allows a plurality of light streams having distinct and finely spaced wavelengths to propagate together, e.g., in a single-mode fiber. DWDM therefore increases a bandwidth for an optical fiber network. Implementations of DWDM include the use of DWDM filters, which can combine (e.g., multiplex) a plurality of separate light streams having finely spaced wavelengths into a single-mode fiber. DWDM filters can also separate (e.g., demultiplex) a combined light stream (e.g., a multiplexed signal) exiting from a fiber into a plurality of separate light streams each having one or more distinct, spaced wavelengths.
Typical optical networks using DWDM include erbium doped fiber amplifiers (EDFA's). When a multiplexed optical signal propagates through an EDFA, ideally each light stream is amplified independently without interaction among the propagating light streams.
An erbium doped fiber (EDF) is a form of a single-mode fiber, having a core that is heavily doped with erbium. Conventional EDFA's include a pump laser that provides a pump light to the erbium doped fiber to provide amplification. For example, when pump light at 980 nm or 1480 nm is launched into an EDF, erbium atoms absorb the pump light, pushing the erbium atoms into excited states. When stimulated by light streams, for example an input optical signal having wavelengths in a C-band (1528-1570 nm) or an L-band (1570-1620 nm), the excited atoms return to a ground or lower state by stimulated emission. The stimulated emission has the same wavelength as that of the stimulating light. Therefore, the optical signal is amplified as it is propagating through the EDF. Furthermore, the EDF typically amplifies all received light streams regardless of wavelength.
Conventional EDFA's allow optical signals to propagate a long distance in an optical fiber network without using electronic repeaters. An electronic repeater is an electrical amplifier module that typically includes a light detector, an electrical amplifier, and a light emitter, e.g., a laser diode. Using an EDFA in an optical network allows amplification to be performed without converting light to electricity, amplified in an electrical domain, and then converting electricity back to light.
Typically, a gain provided by an EDFA is not uniform across wavelengths of the optical signal. A wavelength dependent gain (or the spectral gain) is a characteristic of the doped material in an EDFA. Additionally, the wavelength dependent gain changes shape for different average gain values (e.g., how the gain changes with wavelength varies according to the average gain value being provided).
A wavelength dependent attenuating filter can be designed to compensate for the wavelength dependent gain of an EDFA at a specific average gain value such that the combined effect provides an ideally flat gain for all operating wavelengths. Such a wavelength dependent attenuating filter is typically referred to as a gain flattening filter (GFF).
An EDFA using a GFF to provide a spectrally flat gain typically includes two EDF's that are coupled by a GFF. The first EDF amplifies an input optical signal. The amplified optical signal is then filtered by the GFF, which typically attenuates one or more wavelengths of the optical signal. The second EDF then further amplifies the filtered optical signal attenuated by the GFF.
Conventional GFF's are configured to provide a flat gain for a specific average gain value only. If another average gain for the EDFA is required, a pump power supplied to both EDF's must be changed accordingly. However, for the new average gain, the spectral gain will be no longer be flat. A GFF alone cannot provide a variable flat spectral gain for different average gain values. Therefore, one compensation technique is to include a variable optical attenuator (VOA) element together with the GFF between two EDF's. To set a new average gain, the pump power is changed and the VOA is configured to provide a specified attenuation. The combined effect of the pump power and the VOA can provide a flat gain for the newly selected average gain.